1. Field of the Invention
The present invention relates to frequency mixers, and more particularly, to a frequency mixer directed to improving linearity.
2. Description of the Background Art
For frequency conversion in a radio communication transceiver, a frequency mixer is employed for converting, for example, an RF signal (radio frequency signal) into an IF signal (intermediate frequency signal).
This frequency mixer functions to multiply two signals of different frequencies to generate a signal based on the multiplied result, i.e. a signal having a frequency of the sum and difference of the frequencies of an RF signal and a local oscillator signal. By taking out only the signal that has a frequency of the difference, the IF signal can be obtained.
As used herein, a local oscillator signal refers to a signal output from an oscillator that generates a signal having a frequency substantially equal to the value of frequency-conversion from an IF signal to an RF signal in a radio communication transceiver.
A conventional frequency mixer is formed of a transistor receiving an RF signal at its gate for conversion into a drain current (referred to as RF transconductor), a differential pair including two transistors, receiving differential local oscillator signals at their gates for conversion into a drain current (referred to as local oscillator signal differential pair), and a load to convert the drain current of the transistors in the local oscillator signal differential pair into a voltage to obtain an IF signal.
The drain of the transistor in the RF transconductor is connected in common with the sources of the two transistors in the local oscillator signal differential pair. Furthermore, two loads are connected to the respective drains of the two transistors in the local oscillator signal differential pair.
In accordance with such a configuration, the transistor of the RF transconductor converts the RF signal into a drain current. The current path thereof is switched by the two transistors of the local oscillator signal differential pair in response to the local oscillator signal applied to respective gates, whereby the multiplied result of the RF signal and the local oscillator signal appears as the differential IF signal at the two loads to be output.
Furthermore, the frequency mixer is configured to allow the drain current of the transistor in the local oscillator signal differential pair to be reduced by compensating for a portion of the drain current of the transistor in the RF transconductor through a constant-current source (for example, refer to IEEE J. Solid-State Circuits, Vol. 35, No. 12, pp. 1927-1933, referred to as Document 1 hereinafter; and IEEE J. Solid-State Circuits, Vol. 38, No. 1, pp. 30-42, referred to as Document 2 hereinafter).
Thus, the frequency mixer disclosed in Documents 1 and 2 can reduce the drain current of the transistors in the local oscillator signal differential pair that is required to conduct a current flow to the drain of the transistor in the RF transconductor.
Even in the case where the gate-source voltage of the transistor in the local oscillator signal differential pair is smaller, i.e. when a local oscillator signal of smaller amplitude is input, the frequency conversion operation described in the foregoing can be carried out, allowing the linearity in the local oscillator signal differential pair to be improved, i.e. improvement of linearity in the relationship between the local oscillator signal (input) in the local oscillator signal differential pair and the drain current (output) of the transistor in the local oscillator signal differential pair.
At a point in time where the momentary voltage of the RF input signal, when having a large amplitude, is small in the frequency mixer disclosed in Documents 1 and 2, the gate voltage of the transistor in the RF transconductor will become extremely low. Therefore, there are occasions where the drain current of that transistor (the output current of the RF transconductor) becomes smaller than the current of the constant current source. In order to reduce the current flowing to the drain of the transistor in the RF transconductor (input current of the RF transconductor) at this stage, the potential at the connection node among the drain of the transistor in the RF transconductor, the constant current source, and the sources of the transistors in the local oscillator signal differential pair rises. In response, the gate-source voltage of the transistor in the local oscillator signal differential pair is reduced, whereby the transistor in the local oscillator signal different pair is turned off such that the drain current of that transistor becomes 0. As a result, no current will flow to the load connected to the drain of the transistor in the local oscillator signal differential pair, and the IF signal will be fixed in voltage.
Thus, the frequency mixer disclosed in Documents 1 and 2 had the disadvantage that the linearity of the local oscillator signal differential pair is degraded when the amplitude of the RF input signal is large.